1. Field of the Invention
The present invention relates to methods of removing filter cake deposited by well drilling or servicing fluids from well producing zones.
2. Description of the Prior Art
The use of special fluids for drilling or servicing hydrocarbon producing formations penetrated by well bores is well known. The drilling fluids are utilized when well bores are drilled into producing zones to minimize damage to the permeability of the zones and their ability to produce hydrocarbons. Servicing fluids are utilized when completion operations are conducted in producing zones and when conducting work-over operations in the zones. The drilling and servicing fluids deposit filter cake on the walls of the well bores within the producing zones which prevents the drilling and servicing fluids from being lost into the producing zones and prevents solids from entering the porosity of the producing zones. After the drilling or servicing of a producing formation has been completed, the filter cake is removed prior to placing the zone on production.
The removal of the filter cake from a producing formation has been accomplished in the past by including an acid soluble particulate solid bridging agent in the drilling or servicing fluid for bridging over the formation pores. The filter cake formed by the drilling or servicing fluid has heretofore been contacted with a strongly acidic solution so that the acid soluble bridging agent is dissolved. The acidic solution has been allowed to remain in contact with the filter cake for a period of time sufficient to dissolve the bridging particles. In spite of efforts to avoid it, the strongly acidic solution has often corroded metallic surfaces and completion equipment such as sand screens and caused their early failure. The strongly acidic solution can also be incompatible with the producing zone and cause damage thereto.
Filter cake often needs to be removed from the walls of open hole well bores penetrating producing zones in competent sandstone, in screen only open hole completions, in open hole carbonate wells and in gravel packed wells. However, the removal processes employed heretofore have often caused at least some damage to the permeability of the producing zones.
When the producing zone penetrated by a well bore is in an incompetent formation, i.e., a formation which contains loose sand and fines, gravel packs are commonly placed in the well bore adjacent to the incompetent producing zone to prevent the migration of the loose sand and fines with fluids produced from the zone. In forming a gravel pack in a producing zone penetrated by a well bore, a sand screen is typically positioned in the well bore adjacent the producing formation. A crossover tool which supports a packer is connected to the sand screen and to a pipe string. The packer is set by pipe movement or other procedure, and the crossover is a subassembly which by pipe movement or other procedure selectively allows fluids to flow from inside the pipe string to the outside of the sand screen, i.e., to the annulus between the sand screen and the walls of the well bore, and from the outside of the sand screen below the packer through the sand screen and the crossover to the annulus above the packer between the walls of the well bore and the pipe string. In operation, gravel, usually graded sand, is mixed with an aqueous carrier liquid, and the liquid containing the gravel is pumped down the pipe string through the crossover into the annulus between the walls of the well bore in the producing zone and the sand screen therein. The sand screen prevents the gravel deposited in the annulus from flowing through the sand screen but allows the carrier liquid to do so which is conducted through the crossover and upwardly through the annulus between the pipe string and the walls of the well bore to the surface. In this manner, a gravel pack is produced in the producing zone between the walls of the well bore penetrating the producing zone and the sand screen therein. The well is then produced by reversing the crossover tool and flowing fluids from the producing zone through the gravel pack and sand screen whereby migrating formation sand and fines are separated from the produced fluids and the produced fluids flow upwardly through the pipe string to the surface. The gravel used can be coated with a hardenable resin composition which hardens after placement of the gravel and forms the gravel into a hard permeable mass.
The removal of the filter cake formed on the open hole walls of the producing zone can be accomplished prior to the formation of a gravel pack therein. However, without the filter cake on the walls of the producing zone, the carrier liquid utilized to place the gravel will readily flow into the producing zone making the gravel pack very difficult to place in the producing zone, particularly when the well bore penetrating the zone is horizontal. In addition, the producing zone can be damaged by the flow of the carrier liquid and fine particulate material into the producing zone. In order to prevent the loss of the gravel carrier liquid, damage to the producing zone and/or the collapse of the well bore, the filter cake formed by the drilling or servicing fluid has been removed after the gravel pack is in place. However, the sand screen and gravel pack often prevent access to the entire filter cake and many post gravel pack treatments to remove the filter cake are only partially successful or not successful at all.
Thus, there are needs for improved methods of removing filter cake from well producing zones after gravel packs have been formed therein.